1. Field of the Invention
The invention relates to a rear mixer ejector for a turbomachine intended for propelling supersonic civil aircraft.
The noise level caused by the exhaust gases from the nozzle of a turbojet engine increases as the gas ejection speed increases. This noise, known as jet noise, is very noticeable in the case of a single flow, or straight, turbojet engine.
In order to reduce the noise level of jet aircraft near airports and at low and medium heights their engines must be provided with means for attenuating exhaust gas noise, and an effective technique for reducing jet noise is to mix the primary highvelocity hot gas flow from the gas generator, or combustor, of the turbojet engine with low-velocity cold air delivered from a secondary flow path outside the primary flow path in order to produce a relatively uniform mixture of intermediate velocity at the exhaust nozzle exit.
2. Summary of the Prior Art
Various fixed means for achieving this mixture are known, such as organ pipe devices, daisy devices or lobed devices. However, they are permanently present in the flow path of the hot gases leaving the combustor, and this impairs the performance of the engine since it leads to excessive fuel consumption in some flight conditions.
The ejection nozzle of supersonic aircraft has a variable configuration which is adaptable to flight conditions and aircraft speed. Reducing noise from the jet engines of such aircraft is unnecessary during supersonic cruising, descent, approach and landing, since the aircraft flies at a very high altitude when cruising at supersonic speeds, and engine speeds are low during the other flight regimes. During take-off and climbing, however, attenuation of the engine noise is particularly necessary, and temporary cold air feeds must therefore be available for mixing with the hot gases from the combustor in order to lower the hot gas temperature and thus reduce the speed of the gases ejected from the nozzle.
U.S. Pat. Nos. 5,154,052 and 5,291,672 disclose rear mixer ejectors for supersonic aircraft engines wherein the ejector casing has a rectangular cross-section in which the opposed top and bottom walls are provided with variable-geometry convergent-divergent flaps and pivotable mixing devices comprising a number of spaced-apart chutes movable between a deployed position, in which they channel cold air from the outside into the hot gas flow path, and an inoperative retracted position. The chutes each comprise two spaced-apart lateral walls which are of triangular shape and which are interconnected along one of their edges by a transverse wall forming a chute bottom which extends inwards into the hot gas flow path and in the downstream direction when the chutes are in the deployed position. In the inoperative position the chutes are retracted into the thickness of the casing and the chute bottoms close the air inlet apertures which are provided in the casing walls in correspondence with the chutes. This arrangement makes it necessary to increase the casing wall thickness, and results in an undesirable increase in aerodynamic drag and excessive fuel consumption.